Solved with complete phone reset.
I experienced the same issues with the Garmin HRM and cadence sensor both reading null (–) on my iphone XR but this only happened after updating to the new TrainerRoad IOS app, released in March. Prior to that they had been working fine since I began TR 9 months prior.
After trying many different things with the help of TR tech service, I tried resetting my phone and did a restore from a backup that I had created. That still do not solve the issue so I did a full reset and this time setup the phone as a new phone, meaning I had to manually install all my apps and set up email again. It was a bit of a pain but it solved the issue and I once again have heart rate and cadence data.